design - what is microcoded architecture in computer architecture

I want to know what is microcoded microarchitecture of an instruction set architecture (ISA) and why is it used? What is the difference between microcoded architecture and single cycle microarchitecture?

Please clarify with some examples.

Thanks

Answer

Not all of the CPU instructions have the same complexity. When instructions are particularly complex (like some x86 instructions), it is more convenient to implement them as a series of elementary instructions that are contained in a very fast internal memory. When a microcoded instruction is executed the microprocessor split it into a series of simple hardware coded instructions and execute them in series.

So microcode is used in more complex instructions, while simple instructions are directly executed. For example today Intel microprocessors uses microcode in order to execute some old legacy instructions. The x86 microprocessors, that on the paper are CISC, internally uses an architecture that is more like RISC in order to boost the performance and they offer support to the legacy instructions through microcode.

You can find many RISC CPUs that don't use microcode. This is essentially due to their reduced instruction set, that makes more convenient implement all the instructions in hardware. An example of microprocessor that doesn't use microcode is the MOS 6502.